arrow2_convert primitive (de)serialization performance issues

[teh-cmc]

Serializing and (maybe?) deserializing primitive types through arrow2-convert's APIs is much slower than doing so using raw arrow2 arrays directly.

See #1742 for detailed benchmarks.

[teh-cmc]

• Thankfully only really impacts serialization, not deserialization
• Need to introduce specialized fast paths for serializing control & component columns that are backed by primitive types, which shouldn't be much of an issue

[teh-cmc]

• The fast path for control columns is straightforward and has been added in Datastore revamp 2: serialization & formatting #1735, although:
  • We still need a fast path for utf8 (entity_path)
  • We still need a fast path for timelines & timestamps, but requires TimePoint should be serialized as multiple columns rather than a single list-column #1757 first
• The fast path for component columns is much more complicated to get right since everything is erased at that point
  • Gave it a timeboxed half hour, will have to get back to it later

[teh-cmc]

Closing: we're moving away from arrow2-convert.